Polybenzazole fibers have not less than twice the strength and elastic modulus of polyparaphenylene terephthalamide fiber, which is a representative super fiber currently on the market, and is expected to be a super fiber of the next generation.
It is conventionally known that fibers are produced from a solution of polybenzazole polymer in polyphosphoric acid. For example, U.S. Pat. No. 5,296,185 and U.S. Pat. No. 5,385,702 disclose spinning conditions, WO94/04726 discloses water washing and drying methods, and U.S. Pat. No. 5,296,185 discloses heat treatment methods.
However, the compression strength of high strength polybenzazole fibers according to the above-mentioned conventional production methods is generally 0.4 GPa at most. This has become a threshold in applying a polybenzazole fiber to composite materials used in aircraft and the like.
Thus, the present inventor has conducted intensive studies in an attempt to develop a technique- to easily produce a polybenzazole fiber having an ultimate elastic modulus as an organic fiber material.
As a means to realize ultimate physical property of the fiber, rigid polymers such as so-called ladder polymer have been considered. However, such rigid polymers lack flexibility, and therefore, it is essential to use a linear polymer to afford soft touch as organic fiber and workability.
As shown by S. G. Wierschke et al. in Material Research Society Symposium Proceedings Vol. 134, p. 313 (1989), it is polyparaphenylene benzobisoxazole in cis-form that has the highest theoretical elastic modulus in linear polymers. This conclusion was also confirmed by Tashiro et al. (Macromolecules. vol. 24, p. 3706 (1991)), wherein, of polybenzazoles, cis-form polyparaphenylene benzobisoxazole showed a crystalline elastic modulus of 475 GPa (P. Galen et al., Material Research Society Symposium Proceedings Vol. 134, p. 329 (1989)), and was considered to have an ultimate primary structure. Accordingly, it is a theoretical conclusion to use, as a polymer, polyparaphenylene benzobisoxazole as a material to achieve an ultimate elastic modulus.
The polymer is fiberized according to the methods described in U.S. Pat. No. 5,296,185 and U.S. Pat. No. 5,385,702, and the heat treatment is performed according to the methods proposed in U.S. Pat. No. 5,296,185, but a yarn obtained according to these methods has a compression strength of 0.4 GPa at most. Therefore, the necessity of research of modification of these methods was keenly realized and the present inventor has found that the expected physical properties can be easily achieved industrially by the method shown below.